Take the 3mm and a beanie. If you get cold put on the beanie.
I've been diving in SE Asia for the past number of winters and normally my 3mm is enough but in Bali, I not only had to layer underneath with a Sharkskin but my beanie wasn't enough. Fortunately, I had my 3mm bibbed hood along.
In North Sulawesi, I've generally been comfortable in my 3mm and adding the beanie on multi-dives.
Thailand, for me was in the Gulf but in October and I was comfortable in my 3mm, no beanie.
I've been diving in SE Asia for the past number of winters and normally my 3mm is enough but in Bali, I not only had to layer underneath with a Sharkskin but my beanie wasn't enough. Fortunately, I had my 3mm bibbed hood along.
In North Sulawesi, I've generally been comfortable in my 3mm and adding the beanie on multi-dives.
Thailand, for me was in the Gulf but in October and I was comfortable in my 3mm, no beanie.